Commercial vegetable production, including the production of tomato, is affected by many conditions. The choice of the grower for a certain variety is a determining factor, and forms the genetic basis for the result that can be achieved. In addition, there are many external factors that influence the outcome. Growing conditions like climate, soil, and the use of inputs like fertilizer play a major role. In addition to this, the presence of pests and diseases also affects the total yield that can be reached.
Many diseases in tomato have been acknowledged a great while ago, in the early years of tomato cultivation. Breeders have in the meantime identified resistances to a great number of these diseases from various sources and have incorporated them in their products. Examples of these are resistance to Tobacco Mosaic Virus (TMV), which can infect a wide range of vegetables and other crops; Fusarium oxysporum f. sp. lycopersicum, and Cladosporium fulvum or “tomato leaf mold”. Nowadays resistance against those diseases is more or less standard in all commercial tomato varieties. Whenever a new strain or related disease shows up, the search for new sources of resistance starts all over again. Knowledge of the disease and of the existing resistance to a possibly related form helps to determine new resistance sources relatively quickly. For some diseases however, and especially for pests, it is very difficult or has until now been impossible to develop material with a high level of resistance. Especially when resistance mechanisms are very complex, and rely on several genes that interact with each other, the challenge to develop a good level of resistance can be really high.
In addition, sometimes completely new diseases appear that are not related to any of the already known ones. For these, there is no indication yet what could be the most likely germplasm from which resistance can be developed. Neither is the resistance mechanism known, which also makes the development of a new resistance more complicated. As an additional complicating factor, a good bio-assay is needed to compare resistant plants with susceptible material. When little is known of a new pathogen, first the way in which it can infect a tomato plant has to be determined. A bio-assay that does not correlate with the conditions in the field of a grower could result in contradictory or unsatisfactory results. Too mild or too strong inoculation during a test will not generate useful material to work with for development of a resistant tomato plant in practice. In the end, the ultimate test is whether a resistance holds under a grower's conditions.
In 1999, a new virus occurred in commercial tomato production in Europe, especially in greenhouses. This virus could spread extremely quickly through a whole field, and neighbouring growers were easily affected. The virus was soon identified as Pepino Mosaic Virus, belonging to the Potex Group, which is characterised as highly infectious and persistent.
Pepino Mosaic Virus (PepMV) was first identified in 1974 on pepino or pear melon (Solanum muricatum), a South-American crop, on plants originating from Peru. It was at that time determined that tomatoes, and related wild species, could be infected, but without showing symptoms.
It is not yet determined how the virus could suddenly appear in European tomato productions, and later on also in e.g. Canada and the United States. Several different PepMV genotypes are identified and distinguished, among which are: LP, the original one from Peru; EU, from European greenhouses; CH1 and CH2 from Chile; and US1 and US2.
The PepMV isolates that are present in the commercial tomato crops are more virulent in tomato than isolates that are taken from a pepino crop, suggesting that the virus has genetically adapted. PepMV spreads very easily mechanically, through the usual activities that are done while working in a tomato crop. Very often therefore infected plants can be seen subsequently in a row. Also tools, clothes, etc. stay capable of transmitting the virus for several weeks, and PepMV can stay in dry plant material for as long as 3 months. It is very difficult to get rid of the virus once it has infected a tomato production.
Symptoms of PepMV are various and largely depend on the plant stage during infection, plant variety, plant vitality, and growing conditions. Sometimes symptoms are hardly visible, but the main symptom expressions include plants with ‘nettle heads’—grayish, spiky plant tops—, stunted heads, chlorotic leafs or leaf spots, and uneven ripening, marbling, and blotching of the fruits. Symptoms are most apparent during fall and winter, under low light conditions and lower temperatures.
Losses of tomato production due to PepMV can also vary significantly, depending on the circumstances. In heavily infected crops, losses can probably reach up to 20%. The presence of other pathogens, for example Verticilium spp., can strongly influence the yield reduction as well.
Due to the very easy spread of PepMV, strict hygiene protocols have been implemented in many countries and by many growers. Since it is assumed that PepMV can also be transmitted through infected seeds, hygiene protocols for seed production and seed cleaning are also very strict.
Within the EU, tomato seed has to be free from PepMV when it is imported or traded. EU members are required to do surveys to determine the absence of the virus on tomato seeds.
Since it was found to be very difficult to eradicate the virus after it had infected a commercial growing, nowadays many growers rely on ‘cross-protection’: inoculation of the crop with a mild PepMV isolate, to prevent the severe symptoms that are caused by aggressive isolates. This system however brings several risks. The combination of certain mild with certain aggressive isolates, especially when they originate from different genotypes, can enhance instead of diminish symptoms (Hanssen et al, Plant Pathology 59, 13-21 (2010)). Since it is not known in advance which aggressive isolate will occur in a certain area or certain season, the possibly harmful combination cannot be prevented. In addition, it is not even always clear which mild isolate is being used because the identification is rather difficult.
Another risk of combining viral genotypes is the possibility of genetic recombination between the strains, which can result in new and potentially even more devastating virus isolates (Hanssen et al., European Journal of Plant Pathology 121, 131-146 (2008); Hasiow-Jaroszewska et al., Acta Biochimica Polonica 57, 385-388 (2010)).
Although the search for sources of resistance to PepMV in tomato has been intensive from the start, until now no resistant Solanum lycopersicum plants are available. The genetic makeup of the resistance, and the bio-assay for screening, are so complex, that no reports or notices of cultivated tomato material with PepMV resistance are known. Durable resistance has only been found in Solanum ochranthum, which cannot be crossed with cultivated tomato.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.